Cytochrome P450s are involved in the metabolic activation of chemical carcinogens. Marked species differences exist in the catalytic activities of different P450 forms. To study human P450 in an intact organism, transgenic mice are being made that contain human P450 genes. The human CYP1A2 gene was isolated from a gene library and characterized by sequencing. This gene encodes an enzyme that is known to metabolically activate arylamine carcinogens and heterocyclic arylamine carcinogens. The CYP1A2 gene promoter was excised and replaced by the murine elastase gene promotor that functions in pancreatic exocrine cells. The recombi- nant DNA was injected into mouse eggs which were then implanted into pseudopregnant mice. Transgenics were identified by analysis of tail DNA from offspring by hybridization with the human CYP1A2 cDNA. Among six mice having human DNA, two were found to express CYP1A2 protein and activity. One line having a high level of expression is being thoroughly characterized. We found that although high levels of CYP1A2 protein are made, the activities obtained toward the substrate 7-ethoxycoumarin are lower than expected from the purified enzyme produced using cDNA expression in baculovirus. Therefore, we are preparing a double- transgenic mouse that contains the NADPH-P450 oxidoreductase gene driven by the elastase promoter. These mice should be useful in determining the role of human P450 in metabolic activation of carcinogens and toxins.